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4 Boeing 707 for $50,000. Is that possible?
I'm helping a friend with an article on some issues in my country, mostly related to corruption. His English isn't very good, so here I go
One of the cases he's analyzing is the sale of 4 Boeing-707 airplanes for $50,000 . These airplanes couldn't fly but according to public information, they had their avionics, each of them has 4 working (not suitable for flying) Pratt & Whitney JT3D engines and one of them had a working intelligence/surveillance system (this was a very complex subject and now government says they'll try to keep this for training). Airplanes were for military use, both passengers and cargo.
Even considering selling them for scrap or replacement parts, is $50,000 a realistic price? For example, is there a place I can find prices for the parts, eg engines, landing gear and such?
EDIT: this is in Argentina, newspaper article (in Spanish).
Some data translated from article:

Approximate cruising speed: 977 km / h
Service ceiling: 11,000 meters
Approximate range: 8690 km
Empty weight: 62,500 kg
Load capacity: 94,000 pounds
Maximum operating weight: 335,600 pounds
It could carry up to 180 passengers

They are four-engine jet aircraft, Boeing brand.

Primary function: Transport of passengers and cargo, as the case may be.
Wing type: low implantation. Undercarriage: type retractable tricycle with 10 armed wheels. They have four Pratt & Whitney engines (turbines).
Length: 44 meters. Span: 47 meters. Tail height: 13 meters.

3 of the airplanes were bought by an US company specialized in airplanes' instrumental and parts, 1 was bought by a local party.

Where they are located may be a big part of a low price, maybe they can't be scrapped in-place, or there is a contingency for moving them in a certain period of time. There isn't a big demand for 707 parts. Maybe the contract says that they have to be refurbished for training, etc. There are too many questions without reading the actual purchase offer or sell agreement.

USD 50,000 each?

No, all 4 for 50,000

Airlines are scrapping Airbus A320 and Boeing 737NGs... and replacing them with brand new versions of the same plane. The condition and installed equipment is what determines the value. Without some detailed knowledge, you can't say whether it's a good deal or not. Even a modern fly-by-wire airplane can be worth no more than scrap metal.

Make sure that the $50,000 is 'free and clear'. You may be liable for outstanding parking charges etc!

Would be cool time building a flight simulator out of one of them, with all switches and controls already in place, but this (and others similar) "creative" approaches probably need to much work to be viable commercially.

It is true that the 4 airplanes were bought in $ 50,000.00 in a public auction.
They were bought for scrap and, if possible, for selling some spare parts like the landing gears or engines.

I know it's true, hence why I ask if this price is posible.

Reading the article and by today argentine peso each airplane costs11,500 to 14,150 US dollars each. They are not airwoth since 2006 and "some parts are missing", they are parked at El Palomar air base. One was used in 1992 to "eletronic wars" (not with prime 90's eletronics I suppose) but Argentine Air Force will keep it's cabin for training. Seeing the pictures they look like barely worth for scrap.I suppose one can get one ariframe and build an exotic trailer. Looks like you will pay more to transport, store, torn it apart and sell it than buying it

Its surely possibly and there are a few things I can think of off hand,

Missing Log Books: generally speaking if an aircraft is missing its log books you need to do quite a bit of tear down and rework on it to make it airworthy once again. In many cases the cost of this far out weighs the cost of a logbook/airworth plane so not always done. As such this can severely drive down the price of the aircraft making it close to worthless. In line with this, without the log books you can not sell any of the parts for replacement either. With out the ability to properly tag the part they are only worth their weight in scrap. Considering they seem unflyable and are fairly large to chop up and ship scrapping it can also become a costly maneuver.
The tax man...: Airplanes (and all manner of vehicles) often change hands for strange dollar amounts due to various tax regulations. That does not look like the case here but it does happen.
Simply Outdated: keep in mind the 707 is a fairly old plane by anyones standards these days. While it may have avionics chances are they are no newer than when it was parked there to collect dust, like it or not the best LORAN C unit in the world may have cost a fortune to install back then but is worth next to nothing now.

These things dont exactly go up for sale all that often but here is a pretty nice one listed for 6.2 Million and here is reference to one listed for only 700K. Please keep in mind that there are countless variables for this kind of thing that could drive the price both up and down drastically.

Actually $50,000 is a good price, at least for scrap.  If the empty weight of each jet is approx 130,000lbs and aluminum sells for roughly 25 cents on the pound of scrap, that gives a scrap value for each jet at roughly 32,500 USD or about 130,000 USD for all four aircraft, netting you a profit of 80,000 USD on the sale.  DO NOT BUY THE AIRCRAFT FOR FLIGHT USE; my guess is they’re worn out airframes and engines and would cost in the millions to refurbish them for that role.

You can't directly compare an airplane to the price of scrap aluminum. The problem is an airplane consists of many other metals, like stainless steel and magnesium, some of which are even radioactive. Plus hazardous materials like hydraulic fluid, lead, and even depleted uranium that must be removed and safely disposed of. Finally, aircraft use different alloys that contain different elements (such as copper) that aren't desirable in the common grades of aluminum.

6061 and 7075 alloys are used in all kinds of products other than airframes.  The rest of that is handled with the recycling process.

That's a common misnomer. By weight, an aircraft is mostly alclad 2024 followed by some 7075. 6000 series alloys are rare. The issue is that none of these are suitable for casting: the majority of scrap is 300 series, used for cans and engine blocks. If you show up to a scrapyard with tons of non-cast non-6061 Al, they will pay you less because of the different composition and thus the more work the foundry has to do to adjust.

No it’s not.  6061 is one of the most common alloys out there for aircraft structures- been used since the DC-3 program.  Also the cost estimates I give for scrap are on the lower end of the spectrum.  It’ll sell for that.

Once again you're wrong. The DC-3 was built with 24S, which is the old name for 2024. On the second to last paragraph, the linked document even lists the 2000 series and 7000 series used on the Boeing 777. If you're going to make assertions, you need to back them up with supporting evidence, instead of repeating the incorrect myth that airplanes use 6061.

Dude I work for 10 years in that industry designing aircraft structures, including on the G650 airplane which used 6061 aluminum alloy for its primary structure.  Boeing made use of it extensively as well

According to the article, they were classified as unrecoverable, damaged, had missing parts, didn't fly since 2006, and the EW system dated back from 1984.
Old aircraft (and other large hardware, like ships) get auctioned off all the time. Since the buyer is normally also responsible for getting it out of the field to wherever they want it, their actual costs are a lot higher than $12,500 per aircraft.
There's certainly more parts (in however little demand, as these are outdated) and scrap value in the planes, but cutting them up, finding the few parts worth something, taking them out, and disposing of what remains also takes money.
Disposal also has a cost to the seller, so sometimes they're willing to let what amounts to scrap for them to go for free, just to be rid of it. In that light, $50,000 is more than nothing.

Although the previous answers definitely provide accurate information in terms of the possibility that this is either for scrap airframes or a fraud deal, I would like to open your perspective to one more type. For certain countries and corporations, trades and sales between two groups that are at least friendly in relations can take part in certain limited "donations" in which a country (say the US) "sells" a set amount of old or outdated military hardware to an allied country with very limited offensive or usually defensive capabilities (think Western-aligned African and South American countries). This is more commonly done with naval or disposable hardware as they are more difficult for these friendly countries to develop/obtain otherwise, and are sold for a symbolic price, even something as low as $1.00 as to make the transfer an official sale. I understand that your friend is investigating something FROM a smaller country (Argentina by no means is needy in the military hardware category) but still country-to-corp transfers like these are not uncommon. The case I wish to point out is the sale of 23 Mig-29's from the recently reunited German Luftwaffe to Poland's air force for a symbolic price of 1 Euro. This is a great example of politically passable and legal transfers for what would seem like criminally low prices.

I would recommend you do this for either scrap, or, if you have time and money, a worthy airliner. As a person who collects jets (Like, old De Havilland Comets), I would pay you in the 10-100 millions to get one. Just an FYI, museums would love on of these for display. I sould a DC 3 to a museum and got shy of 1 million, and it cost me 20k in modern money.

A 707 isn't exactly as rare as a DeHavilland Comet is.  It's not going to command that kind of money.


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Is inverted flight possible under these conditions?
My understanding of flight is that the primary aspect is the angle of deflection, and the purposes of the shape of the wing is to maximise efficiency. Could a commercial airliner with asymmetric aerofoil sustain inverted flight by keeping its nose up, or something similar?
For the purposes of the question, ignore issues with the fuel or engine, and assume that the wing shape is fixed and not specifically built to accommodate inverted flight, including not having flaps or anything in the aileron department.

Yes, based purely on the wing shape an airliner could fly upside down and maintain a straight inverted flight. The lift of the wing is a function of the vertical angle of the airflow, the angle of attack.
Image source
The picture shows an airfoil pointed upwards, but the same forces apply when the wing nose point downwards. Then turn the picture upside down, and you have lift pointing upwards again.
The situation is valid for an asymmetric airfoil as well, the only difference being that it provides lift at AoA zero. Zero lift is provided at a slight negative angle of attack, so the aircraft nose will be further up when inverted, still below the stall angle though.

Thanks. Can you please link me to a paper that talks about this? I am not believed.

The only case I could find of an airliner flying inverted for any period of time was Alaska Air 261. The jackscrew that set the incidence on the horizontal stabilizer stripped out due to shoddy maintenance, putting the stabilizer in a severe nose down position that elevator and trim input couldn't correct. For a minute or two, the pilots did manage to hold the plane inverted in an attempt to counteract the broken stabilizer, but the plane ended up crashing into the sea due to the inability of the pilots to regain full control.
So the answer would be a qualified 'maybe', as it is unknown how much of a contributing factor the inverted flight was to the final descent.

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What replacement wings are available for old F-16s?
Eventually, material fatigue will force any aircraft to either be grounded or have its wings rebuilt, or replaced. And please, correct me if I'm wrong.
For the F-16, if one were to replace its wings, what alternatives are there?
Are wings for the F-16 still made, as they were in the 1970s, or does multiple modern alternatives also exist? Can you buy e.g. carbon fiber wings for the F-16? What about other composite materials?
If not, why not? Is the performance different negligible and not worth the cost compared to the original wing?
As a bonus question: If there are indeed more high performance wings, such as carbon fiber wings available for the F-16, how does the performance of the aircraft overall, and especially in a dogfight get affected? I imagine all such studies are classified, if they exist at all, but if they are public, I'd love to see such a study.

"And please, correct me if I'm wrong." I believe you are very much off-base here. If anything was fatigued it would be the main spar (in the case of the F-16, there are multiple), not the wing.

@Ron Beyer Thanks for that, I didn't know.

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How to calculate required thrust for take-off knowing gross weight and L/D?
I know that many variables play into the required thrust for a take-off and for climb. Most of those specs boil down to a targeted gross weight, wing loading and L/D ratio. Intended flying speed range is also known but probably not even needed for this estimate. Using these specs then I can roughly assume that many specs have been taken into account for a thrust calculations.
For ex. aircraft is gross weight 400 lb with L/D ratio of 10. Assuming a non-accelerating level flight at optimal cruise speed 50 mph in no wind condition it will require at least 400 / 10 = 40 lb of thrust.
Based on above numbers how to make rough estimate of the minimal thrust required for a reasonably short take-off (say within 500 to 1,000ft roll of runway) ?
Note: please keep in mind I am familiar with many other conditions and aircraft specs affecting the required thrust. Yet for 100 years practical aircraft designers used very simple math approach to estimate needed thrust. I personally knew one of old generation designers who did this well in many of his successful designs using simple math dealing with few numbers (he passed away years ago). He only used gross weight, wing loading and targeted L/D ratio chosen from his experience.
Please share if you know the answer.

http://www.dept.aoe.vt.edu/~lutze/AOE3104/takeoff&landing.pdf
This source has been really helpful for me in calculating takeoff time and takeoff distance. In this method, the author uses thrust as an input to calculate takeoff distance, but obviously you could re-arrange the equations to calculate for thrust if you knew the other variables. You will likely need to know the static thrust (zero speed thrust) of your engine and then can calculate for thrust required at takeoff. I would recommend putting the equations into an excel spreadsheet where you can then play with all the inputs until you reach a desired result. Hope this helps!

Thank you Jason for this reference. It calculates from the other end: from known BHP of a chosen engine which is opposite goal to mine. I need to find required thrust and whence power for take-off in order to arrive at what engine power I need for this design. Then I just make the motor I need :) Also those calculations are far from "simple" as they require more decisions to be made on the particular specs. I cast plus for your answer but it only recorded - does not change the publicly shown as I am new here.

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What is the aerodynamic purpose of an Acorn?

This is an IL-62. On the tail of the aircraft,  there is a large cone, protruding out of the vertical and horizontal stabilizer which is called acorn. I am trying to figure out what is the aerodynamic explanation for its presence over there.

Welcome to Aviation.SE! I think this is a well done first question. I can't find any exploded-diagram type views for that, but I'm guessing that it is a housing/cover over the actuators that move the all-moving tail surface up/down.

Thanks! I guess you are right, probably a housing for actuators or antenna.

That is an anti-shock body or Küchemann carrot, an aerodynamic improvement - there is little to fair over ahead of the intersection between the horizontal and vertical tail surfaces.

It's doing one or both of two things:

Fairing in some kind of projection as Ron said, may be an antenna or a mass balance.
Fixing an airflow disturbance at the T intersection.  Bullet fairings are a common band aid for that sort of thing.

Russian designs seem to be big on bullet fairings.

It is a Küchemann body (or Küchemann carrot), a body which lowers the slope of the cross sectional area over the streamwise direction, reducing shocks and wave drag.
Please read this answer if you are unsure why aircraft experience more drag as they approach the speed of sound. By stretching the aircraft body over a larger length, the gradients in the cross sectional area distribution can be made more shallow such that the air has more opportunity to make way for the aircraft. These bodies are a typical feature of 1950's airliners and subsonic bombers, and even tip or overwing tanks can serve as a drag reduction device when properly shaped. Today, the flap track fairings of airliners serve the same purpose.


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How are engine pylon designed?
Currently I'm working on my master's thesis on the topological optimization method of aircraft structures. I'm gonna to explore this method of design on the example of the pylon. Make something like this:

But firstly I need to provide information about conventional methods. Moreover I'll evidently need to compare old and new methods.
That's weird, but I've managed to find much more information about new method, although it is not broadly implemented yet. But seems that it's too few info on the internet about conventional techniques.
To be more specific I'm interested in that red caisson or frame on the picture below. How exactly it (or similar) is designed? Could you suggest some information (books, manuals, tutorials, abstracts... everything) about that. Russian text is also OK.

While this may not be of great help in your case, there are at least two questions on this site: What's that component between the engine and wing? and Why do we use pylons to mount engine on jet airliners? that provide some details.

Are you looking for an answer that explains the stress methods, design criteria, or structural requirements when you say "how exactly it is designed"?

I mean the design parameters. What should thicknesses of caisson shell should be if I know the loads? How many ribs I need? What's their thickness? etc

Like all complex artifacts, aircraft primary structure design is an iterative process.  It begins with aerodynamicists designing the Master Surfaces of the Aerodynamic Envelope.  Structures designers then cope with the space requirements thereafter, although compromises between multiple design teams will often be needed.
A few things will be known.  We will know the intended operating envelope of the aircraft with some intended design limitations in airspeed, maximum Mach no., service ceiling, etc of the aircraft so we can calculate the aerodynamic loads on the pylon.  We also can obtain performance numbers on the engine, so we can determine the thrust loads and moments about the mount.  We also have the design load factors for the aircraft as well as crash and emergency design loads based upon aircraft certification requirements, factors of safety, etc.  From there these external loads can be computed and used in the design requirement as external loads to a 'black box', that is an as of yet determined structural assembly for the pylon.
The exact design of the pylon and associated mounts are going to depend on these figures.  Pylon design uses similar structures from OEM to OEM and its based on their years of experience designing these thing based on both expected mathematical models as well as flight test data.  Unfortunatly the exact details of how and why it's designed the way it is remain a proprietary trade secret of the OEMs and they are not likely to talk about it.  Boeing, like KFC, has a secret recipe for its wings and is not freely released to the public.  The same is true for its pylons, empennage, etc.
The basics are most new pylons will be based upon this proprietary structural arrangement used in previous designs.  The design loads and basic configuration of the parts an assemblies going into the pylon will have been calculated by a stress engineering group into sizing for the geometry of the parts to be designed.  The first pass at the assembly design will be done by the pylon group, then used as a basis by stress for sizing and this iterative process will be repeated until an acceptable design solution is found.
Ground and flight tests will be the ultimate proof of the design and also usually leads to a weight reduction process to reduce part geometry of excess material once the actual flight loads have been verified.
Your question is an interesting one and it may be more beneficial to consult with either a retired employee from an aircraft design company who worked on these structures or a scholar in aircraft design.  Your school could probably forward you on to someone like that and you can consult with them about this.
After doing so, it might be beneficial to take a trip out to an aircraft boneyard and examine an existing engine pylon on an airplane like a 737, 747, DC-10, etc.  Heck, if the airplane is decrepit enough or about to be sold for scrap, ask the manager how much it would cost to amputate an engine pylon (minus the engine!) and take it back with you for the Master's project.  You could examine it and build CAD and FEA models of the unit, then test these using expected loads for the aircraft and engine in question.  While this might not answer your immediate questions, it will provide your project with a treasure trove of information about how and why an engine pylon is designed the way it is.  You can then examine other OEM pylon design and dissect and analyze them very accurately based on this previous experience.
Hope that helps!


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What would happen if a takeoff is not rejected following an elevator malfunction?
A few weeks ago, the University of Michigan basketball team was on a plane that was in an accident.
Link
How bad could this have been?  From my understanding the issue was here:

Is this the kind of thing that is checked before takeoff?  From my understanding they were traveling at nearly 200 mph when the pilot rejected takeoff.
EDIT:  What would have happened if the takeoff wasn't rejected?

Possible duplicate of In an aircraft with tab-flown control surfaces, how can a jammed surface be detected during preflight examinations or flight control checks?

I don't think this is a duplicate, the OP is asking what the outcome would have been if the aircraft had continued to take-off, not if they could have detected it in preflight.

It's not clear to me if they were even capable of rotating for takeoff if the right hand elevator was jammed in a trailing edge down position.

The answer fooot linked answers the part about it being checked before flight

@FredLarson, the article states that “left hand elevator commenced a large nose up movement … with no change in aircraft attitude however.” So no, they clearly were not able to rotate the aircraft.

@JanHudec: I didn't notice that statement. Thank you.

NTSB report of event under discussion, for future reference.

There are a number of scenarios that I can see happening, most of which depend on the aerodynamics specific to the aircraft.
Scenario 1: Nose Down Force > Up Force
I believe this to be exactly what happened in this instance. What happens is that the downward deflected elevator overpowers the upward deflected elevator creating more down force than up force. This means that the pilot can't rotate the aircraft into a climb attitude. This seems the most probable otherwise the pilots would have lifted off after V1 and went around.
Scenario 2: Nose Down Force < Up Force
If the pilots were able to get the aircraft into the air, and the asymmetric elevator didn't act like ailerons, they would probably have noticed that the aircraft would easily pitch down but be very difficult to pitch up. They would have noticed that as they got faster, pulling back on the stick would have resulted in less of a pitch up. Depending on the pilots, if they noticed this quick enough they could reduce power and control it for a landing. If they didn't realize what was going on and remained at TOGA power, they may have pitched into a stall.
Scenario 3: Extra Ailerons
If the elevators had enough of a roll effect, it would cause the aircraft to roll. They could have probably countered this with opposite aileron, but depending on the amount of up force versus down force scenario 2 would have taken over, only now they would have difficulty turning. (As per Kevin) The asymmetric deployment with the elevator at the top of the tail may create a yaw component, so the aircraft may also be slipping. This can put a lot of force on the tail, especially a twisting force as the pilots put in rudder to counter the yaw.

What it really all boils down to is that the pilots were probably right in rejecting the take-off, and probably would not have noticed the problem until Vr, which is after V1. Learning how to fly a crippled aircraft is not something you want to do at low altitude. If they did get it up and managed to get it in the air, they probably would have gained as much altitude as possible and spent time learning how to fly it while burning off extra fuel. This is pretty much what happened with United 232, where they had to learn how to fly the aircraft again.
Were the pilots right? Probably, but I believe they really didn't have a choice. I'm not sure that the aircraft would have rotated at all. Once they hit Vr and realized they couldn't get the nose off the pavement, it takes a couple seconds to make the decision to pull the power and hit the brakes. At 200mph you are traveling 293 feet per second. If they took 3 seconds to realize the problem and power down, that is almost 900 feet of runway gone.

I'm confused. Down force is what causes rotation. Did if down force > up force then the aircraft would try to rotate early. Or am I reading something wrong?

I believe scenario 3 may get more complicated. The t-tail configuration means the elevators would generator a torque with the top of the tail as its center. That would put a considerable amount of sideway stress into the vertical stabilizer; they would be slipping at the same time as rolling as well.

@TomMcW I meant nose-down force, not tail force.

@RonBeyer Ah! Now it makes sense

Why all the “probably”? The article clearly states that scenario 1 occurred: “… the left hand elevator commenced a large nose up movement as the aircraft accelerated through 152 KIAS and continued in this position for 5 seconds, aircraft accelerating through 166 KIAS with no change in aircraft attitude however.” 152 KIAS would be about right for $V_r$.

In a heavier, faster aircraft, an elevator failure not noticed until V-r could very well be an unsurvivable scenario.


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What would have happened if the jet engine on my aircraft had ingested caulk from the pavement?
I was recently on a flight - the aircraft was a 737-800 - where I had a good view of the front of the left engine. The situation in the title did not occur, but it seemed awfully close.
After pushback, while the plane was waiting to start taxiing under its own power, I looked at the tiny waterspouts sporadically forming in front of the engine (it'd been raining all morning). And then I noticed a piece of caulk (or tar or whatever it is) was being lifted from between two concrete slabs of pavement.
The strip of caulk looked like it might have been 30cm long and 2cm in "diameter" (it had a more rectangular cross-section, but you get the idea). It was still attached at one end, but it was directly in front of the engine, and its free end was being "tugged" up toward the intake in fits and starts, sometimes standing straight up.
Thankfully, it never went further than that. I considered alerting the cabin crew, but everyone was already buckled in, and I really didn't want to cry wolf on a airplane (so to speak). Still, I was very much considering it just as the plane started moving without any further drama.
As soon as we were in the air, I discreetly told a flight attendant, and she told the pilots. She came back a little later asking for a few specifics, so I imagine the pilots were radioing the airport we'd just left.
And I got a free cup of coffee (yay...)
My question is: What might have happened if that piece of debris had come loose and been ingested by the engine?
I wouldn't expect it to be catastrophic or anything (which is also why I hesitated in informing anyone while it was going on), but I would expect it to be expensive. I.e. it might ground the plane, at least for a short while. Probably some flames out the back. Maybe it'd even mess up the engine by gumming up the works.
I was never worried for my safety or anyone else's. I know engines are tough, and can eat all manner of crap. And even if they tear themselves apart in the process, they contain it. Not that I'd expect something like that happening at low power anyway.
But really: I don't know for sure what would happen. I'm not a pilot or mechanic, just a nerd with a nerd's interest in engines and airplanes.

Edit: Checking the airline's website, it seems the engine would be a CFM 56-7B26, if that's at all relevant.

FWIW, had I been the captain of the flight, and the flight attendant had relayed to me what you said, I would have made an maintenance log entry that a passenger reportedly witnessed some FOD (foreign object debris) enter the #1 engine. That would have caused maintenance to take a quick look at the fan for possible damage. Depending on where I was in the world, I might also have while still in flight asked ATC to relay to the airport the possibility of FOD on the taxiway/runway.

@Terry So, to make sure a good inspection is done, you'd make it sound like I exaggerated? :) Cool with me, actually. Out of curiosity, can you elaborate on the "depending on where I was in the world" part? Would there be a reason to not tell ATC that someone might've seen some FOD? Again, just curious.

I would neither exaggerate or downplay. Actually in my ancient world (retired in 1999), it would be the flight engineer that made the log entry, which he would sign, and the captain would review and sign. Any FOD mention would cause a look at the fan blades. Also, looking at the leading edge of fan blades was part of the the f.e.'s regular pre-flight, and every once in a while they would remark that an engine's fan was showing wear. By that they meant there were places a fan blade leading edge had been smoothed by maintenance.

@Terry Gotcha. It'd make sense to me if there was a bit of exaggeration/rephrasing, though. I only saw something almost enter engine #1 - not something that did enter engine #1. But I'd understand why it'd be beneficial to treat it as the latter, just in case.

Well, not an expert on caulking, but birds go through plane engines sometimes, and they seem to be roughly the same consistency overall (if you account for the dirt and pebbles that are sometimes stuck to caulking.) So it possible they would have the same effect.
If you're curious about what would happen if it's like a birdstrike you can take a look at the Miracle on the Hudson, or this 757 that had a birdstrike on takeoff.  With the 757, they simply went around and landed the plane since they only lost one engine...
Anyway, you folks were still on the ground, so all that would have happened is that the engine would have flamed out while you were sitting there and the plane would need to be taken to service to check for damage.  It would have been annoying to have to switch planes, but it wouldn't have been dangerous to any of the passengers or crew.
Nice of them to give you a free coffee though, considering you may have saved them a few hundred thousand in engine repairs if the next jet in that slot had managed to actually ingest the caulking ;).

Thanks for the answer. I do know what a birdstrike can do, but yes: We were on the ground, idling. And my bet would be that that strip of caulk would be less dangerous than a bird, even at take-off power. Nice to know I was right to not worry about catastrophic engine failure and such. By the way, the jet by the gate next to mine was a Dreamliner with engines that looked like they could suck a house of its foundation. I don't know if it would/did pass over the same spot of pavement after pushback, but... well. I'd like to think that airline owes me a coffee too :)

@Flambino To be fair, the strip may have had stones or wood stuck to it, and small stones can cause all sorts of problems with engines too so...  But yeah, I'd probably go collecting on a cup from the 787 as well, work it for as much free coffee as you can ;)

Yeah, I did think that stones or sand in the caulk would be worse than the strip itself - but still pose no danger to anything except the engine. Really, a lot of thoughts went through my mind including everything I know about jet engines, compressor stages, and the effects of debris going into them. Again, layman's knowledge of the subject, but it all made me worry less, not more. The engineer in me trusts those machines. But yeah, if anyone knows someone working Qatar 787s, know that I like a bit of sugar with my coffee, but I'll settle for black ;)

Also, for whatever it's worth, many bird ingestions don't actually destroy the engine, like the ones in the given examples did. Those were more exceptions than the rule (the double engine failure in the U.S. Airways flight was extremely rare... and is a good reason to remember not to fly through a flock of Canadian Geese.) Bird strikes on engines are actually pretty common. I've had a flight where we had to change planes right before boarding was about to start because the pilots found that one of the engines had taken a bird strike on the previous flight when they were doing the walk-around.

@reirab "Canada geese" is the term, I believe. They're not necessarily canucks ;) Canadian geese would be too polite to cause a double engine failure, I'm sure

@Flambino Both names refer to the same animal. Granted, 'Canadian Geese' is more colloquial, but it's also more grammatically correct, so I prefer it. :)

@reirab Ah, well, there goes my hope for more polite fowl :)

It's been a couple of years since I've flown, but, uh, are passengers paying for coffee now?

@RussellBorogove Sadly, yes. This was an international flight, but in Europe, so just under 2 hours flight time, and short flights don't get free coffee. Of course, it was a lower-cost airline too (thankfully a pretty nice one; not rock-bottom), but some of the "traditional" airlines have started nickel-and-diming passengers for stuff like that too.


text:
Why are high-g requirements so important for training next generation of fighter pilots?
Background
In the development of T-X program, a major step has been the refining of the technical requirements, in particular this "sustained G" maneuver:

The sustained G maneuver shall be flown with a standard configuration (i.e., clean with no external stores), at or above 80% fuel weight (relative to
  maximum fuel capacity), steady state flight, and standard day conditions. The maneuver will begin in level flight (flight path angle no lower than zero
  and no higher than two degrees nose high), wings level (+/‐ 5 degrees of bank), at or above 15,000 feet pressure altitude, and at or below 0.9M. From
  this point, the pilot shall immediately initiate bank and back pressure to achieve the sustained G. The sustained G must be maintained for a minimum
  of 140 continuous degrees. The pilot may begin reducing the load factor and rolling out after a minimum of 140 degrees in order to roll out at
  approximately 180 degrees of turn. The sustained G Threshold:  ≥ 6.5Gs; The sustained G ≥ 7.5Gs.

seems to be quite challenging for the T-100 from Alenia and the BAe Hawk.

Alenia T-100 is still bidding after changing partner.
BAe, which partnered with Northrop Grumman, discarded the Hawk in favour of a clean sheet design.
Boeing/Saab participate with a clean sheet design.
Lockheed proposing the T-50 after discarding a SkunkWork clean sheet design.
Textron Scorpion seems to have pulled out after the new set of requirements.

Trend
The new requirement seemed to challenge the existing design. The Hawk could be considered "old" (first flight in 1974), the scorpion could be considered "simple". The T-100 and the T-50 starting development mid 90s together with international partners (Italy/Russia,USA/Korea) should though be "modern" trainer. While the T-100 seems to be more challenged, Lockheed considered a clean sheet design as well, so was probably not too relaxed about the T-50 performance .
Question
 - Why is this request for maneuverability so important for the next generation of fighters?
 - Should not be other factors like battle-space management more important?

Stealth isn't a cloaking system, once another fighter has visually acquired you, you better be able to dog-fight. There are still a lot of aircraft in the fleet that are fighters and pilots need to be able to transition from one to the next. Its highly unlikely that you'll come out of flight school and move directly into an F-35.

@RonBeyer That stealth technology is not a panacea is quite clear! As if you are a fighter pilot, you better be prepared for "dog-fight" it is just part of what you do. That said, the new set of requirements seemed to have "surprised" many bidders, which makes me think that the industry was designing with not such a high maneuverability performance in mind, while the Air Force is pushing in that direction.
Is there a "technical" reason for that? So a combat scenario, where next generation fighter are required a higher maneuverability than previous generation??

@GHB In bullet list, Alenia and Textron links are reversed.

Two solid reasons: defensive and offensive ACM/BFM remain relevant
I'll address the defensive issue first.  Even if a pilot will never get into a dogfight, the pilot / aircraft still must defeat or evade a surface-to-air or air-to- air missile.  This combines the use of both countermeasures and 3-dimensional maneuvering to cause the guidance system inside the missile to lose it's lock on the aircraft.  To get the turn rate and axis-change-rate needed to deal with modern missiles requires the ability to pull a significant number of g's.  You can look at is as a "purely defensive dogfight" technique.  You fight the way you train, so you need to train for this likely situation.
The offensive high energy, high-g capability has to be retained because in real life, the rules of engagement do not always allow for a BVR (Beyond Visual Range) engagement.  Further that point, even with BVR engagements allowed, if the opponent has good defensive technique versus your missiles you may still get into a dogfight-style of engagement in order to defeat or destroy that opponent.  You have to know how to do that, and train for how to do that.  It is an acquired skill. (The basics of the skill were eloquently laid out by John Boyd, which is where the OODA loop model comes from).
Both the offensive and defensive applications of these maneuvers require skill with and acclimatization with high-g maneuvering.
A related reason to boost the performance of the Trainer
For the last 20 years, it has become fashionable, and sometimes useful, to do a thing called "downloading training" into a cheaper to fly, cheaper to maintain aircraft.  That way you don't wear out the airframes of your very expensive front line aircraft.  (The Navy spent years using the T-2 for spin training for its jet pilots, to include T-2's not used in the Training Command but retained on both coasts as "spin birds").   This concept of "downloading training" sounds nice, but in reality has mixed results.  At the high performance level that front line aircraft operate, some skills are transferable and some are "building block" from one airframe to a more complex airframe. Given how recent this requirement is, I'll see in this requirements crafting something similar to the initiatives that I ran into in the Navy (and in Joint Training) with "downloading training."  The same mind set is at work here: if the spec can be met at a high enough level, the graduate of the advanced jet training will enter the front line units with a higher level of capability.   We successfully down loaded quite a bit of instrument training into the sim in at least one aircraft program I worked on, the T-6.  While that was 15+ years ago, the never ending push to "do it cheaper" has not let up in the DoD. Our efforts to download ACM/BFM into the T-6 for the NFO training pipeline ran into the problem of "not enough energy."  I am not sure if that has since been solved, or abandoned, but I think it's the latter.  (The last time I checked on this was about 6 years ago).

Note: This is an experience based answer, informed by my having both been taught basic one-versus-one maneuvering, and many years later having been deeply involved in training systems development, and training requirements, for US Navy fighter/jet pilots. (They had the same challenges to overcome as their Air Force colleagues in the proposed T-X system).  A more detailed answer would be, roughly, book length.  Chapters 5 and 6 of the link cover Offensive and Defensive BFM basics.
From the introduction to that manual:

BFM INTRODUCTION
  When the P-80 finally became operational in the mid-1940s, WWII was coming to a close, as was seemingly the age of aerial dog fighting. While these aircraft never took part during operations in Europe, they became an integral part of combat operations in Korea. In November, 1950, a P-80 flown by LT Russell Brown shot down a Russian MiG-15 in the world’s first decisive all-jet aerial battle, acting as a reminder that BFM was still a very real possibility in combat operations. With the maturation of aerial warfare evolving into Beyond Visual Range (BVR), the decades that followed Korea exploded with the development and implementation of long range air-to-air missiles that could potentially eliminate the possibility of Within Visual Range (WVR) engagements. During the Vietnam conflict, however, aircrew often found themselves unable to employ their BVR missiles, forcing weapons employment into the WVR arena, ultimately relying on BFM skills to defeat their opponents. The days of Korea and Vietnam are long past, and we have achieved amazing successes with the accuracy and reliability of our current long-range
  missiles.  As we, and our enemies, continue to improve our BVR capabilities, a combination of electronic attack, theater Rules of Engagement (ROE), switchology failures, or a momentary lack of attention to prescribed air-to-air timelines may ultimately bring us face to face with the enemy in a 1v1 engagement.
  (CNATRA P-1289 (Rev. 01-16))

The airframe may change, but the problem remains the same.

Historically the maximum g-loads that fighter jets can take have increased over time. Only just recently the planes are truly 9g capable and can retain those forces for a longer period of time (except maybe F-35). To best use the capabilities of the aircraft, it makes sense to pull higher G-forces. Better anti-g suits would also make higher g-forces possible for the pilot.
Another reason might be that Stealth mostly only helps you against a less developed opponent. BVR-missiles are quite easy to evade for modern agile fighters and now that other countries are developing stealthy planes as well a shift to WVR-combat seems likely. The dream of making dog fights unnecessary failed once before when it was decided that fighters don't need a cannon when missiles first appeared. I believe it might be similar with Stealth technology.


text:
What are the differences between an OBS, a CDI, a Heading indicator, and an HSI?
What are the differences between an OBS, a CDI, a Heading indicator, and an HSI? As far as I can tell they all use VORs. Why would 4 different version of the same instrument be invented? Is the HSI the most advanced of the 4 because it shows the most detail?

Some are GPS or glideslope coupled and do not necessarily use just VOR's. For example the second one also has a glideslope indicator and is GPS coupled (Garmin). The third one does not use a VOR/GPS or glideslope, its a gyro-driven heading indicator device.

just say "yes" to backcourse localizer approaches with an HSI

That's not exactly 4 different versions of the same instrument: The third one in your image is just a directional gyro (heading indicator) - it has no VOR/ILS functionality.

You forget dual needle RMI

Check out this manual from the FAA on flight instruments

Heading Indicator:

The heading indicator is fundamentally a mechanical
instrument designed to facilitate the use of the magnetic
compass. Errors in the magnetic compass are numerous,
making straight flight and precision turns to headings difficult
to accomplish, particularly in turbulent air. A heading
indicator, however, is not affected by the forces that make
the magnetic compass difficult to interpret.

Heading Indicators are not VOR related. They simple provide an alternative to the compass and are generally used when the compass reading may be inaccurate (non straight and level flight). Due to their functionality they need to be periodically aligned with the compass.

Course Deviation Indicator:

A course deviation indicator (CDI) is an avionics instrument used in aircraft navigation to determine an aircraft's lateral position in relation to a course. If the location of the aircraft is to the left of course, the needle deflects to the right, and vice versa.

The OBS (Omni Bearing Selector) knob is on the CDI, it is not its own instrument. The knob allows the selection (via the outer ring) of the radial (course) you wish to track from a VOR ground station. When tracking a radial you only track left/right deviation and thus only a single needle is required (the first picture you have). For instrument flying you also need to track the glide slope for an approach and thus vertical deviation may also be displayed on a CDI. When on an instrument approach the CDI is used to track the left/right deviation from the runway (not a VOR).

Horizontal Situation Indicator (HSI):
Is a CDI and Heading Indicator packed into one unit. it provides both left right deviation along with glide slope information as well as heading indication in turns. Many HSI's are coupled to auto pilots and may allow course selection via a heading knob if they are connected to do so. The HSI is not any more advanced than the sum of its counter parts but it is more complex and to a new pilot may be hard to read.

You should add the first image from the question too. It is also a CDI, but without glide-slope (and probably older design).

These 3 things all make use of a Very High Frequency Omnidirectional radio fix.(a VOR). A VOR may or may not have a DME (Distance Measuring Equipment) different symbols on the map.

CDI (course deviation indicator) is like the gauge found in the Cessna that locks on to a fix when you tune your NAV radio to that particular fix.
HSI Horizontal Selection Indicator does exactly the same thing but the design of the instrument is different like in a Beechcraft Baron. It still works off of the NAV radio
OBS button on a GPS unit Omnidirectional Bearing Selector turns the GPS into a VOR tracker just like above but you do not have to tune the radio as it works off of the GPS.
All three only work in the horizontal plane. To lock onto a glidescope you need an ILS (Instrument landing System) fix which does vertical and horizontal navigation.

You are overlooking GPS vertical guidance. RNAV/LNAV do not use any ground based input, with lateral & vertical guidance down to 200 feet above the runway.  With ILS  one can go even lower.  HSI in my small plane is GPS driven.

it is LNAV/VNAV and the vertical guidance is only a nice calculation so you can smooth out the step-down fixes, the pressure altimeter is still the controlling primary instrument.
To get a real GPS vertical glidepath, and 200 feet agl with GPS you need LPV performance which requires either WAAS (wide area augmentation system), or GBAS(LAAS) which is a local ground based augmentation system that corrects GPS signal errors. The LPV and LNAV/VNAV designations are more than signal accuracy, they are totally different approaches with different lateral and vertical obstacle clearance surfaces.


text:
What is the difference between regulatory and nonregulatory airspace?
The first paragraph of the FAA PHAK Chapter 14 - Airspace says:

The two categories of airspace are: regulatory and
  nonregulatory. Within these two categories there are four
  types: controlled, uncontrolled, special use, and other
  airspace.

It never says anything further about regulatory and nonregulatory. So what do these terms mean and what are their differences?

What country/jurisdiction is this for. This answer will vary greatly depending on the context its being asked. (I see the FAA tag so I assume US, is that correct)

Correct, for the US

Non-regulatory airspace is MOAs, Warning Areas, Alert Areas, and Controlled Firing Areas (source 1), (source 2).
Other information should be located in the AIM Ch 3-4, Special Use Airspace. Specifically 3-4-1(c)
Basically non-regulatory areas mean that the FAA does not make regulatory enforcement in those areas due to special (military) operations. Usually the non-enforcement is only for participating aircraft, and generally recreational/commercial aircraft would not fall under any non-regulatory rules.
For example, in an active MOA with aircraft practicing low-altitude speed runs, the FAA would not enforce the 250 knot speed limit on participating aircraft.
This is in the AIM, Ch 3 Section 4, Part 5(b)

Military pilots flying in an active MOA are exampted from the provisions of 14 CFR Section 91.303(c) and (d) which prohibits aerobatic flight within Class D and E surface areas and within Federal Airways. Additionally the Department of Defense has been issued an authorization to operate aircraft at indicated airspeeds in excess of 250 knots below 10,000 feet MSL within active MOAs..

For another example, consider warning areas, which are areas extending 3 miles from the coast. Since this area may include international waters (think of the area between Alaska and Russia), the FAA can't really regulate operations in all these areas. The US Navy also controls some of these areas and "shares" jurisdiction with the FAA.

I believe a TRSA is non-regulatory as well.

@RetiredATC you are entirely correct, which goes to show how incorrect this answer is. TRSA airspace (nonregulatory/nonrulemaking) is almost completely composed of Class E airspace (regulatory/rulemaking) and of course the FAA may enforce rules there.

Regulatory vs Non regulatory has nothing to do with enforcement. What it pertains to is how the airspace is created.
Regulatory airspace must go thru the rule-making process before it can be designated. Each type of regulatory airspace has a corresponding FAR section describing the conditions under which it can be designated and allowable parameters. The FAA must then publish the airspace proposal in the federal register, accept, an evaluate comments before designation of that airspace.
Non regulatory airspace can be designated by the FAA without going through the rule-making process.

Great answer; could you possibly provide some links or citations to back this up?  That would improve the answer even more.

Chapter 3 of FAIR AIM clearly makes the distinction

In the US, the airspace is categorized into regulatory and non-regulatory. According to FAA AIM

Regulatory airspace includes Restricted, Prohibited and various classes (Class A, B, C, D, and E) airspace- These are the areas where FAA regulations are in place.
Nonregulatory airspace includes MOAs (military operations area), warning areas, alert areas, and controlled firing areas.

The non-regulatory airspace are the regions in airspace where the FAA cannot enforce regulations due to various reasons (legal and practical).
For example, the FAA cannot legally restrict aircraft as it lies outside of U.S. airspace. In case of MOA, VFR aircraft are not restricted, though they are encouraged to remain outside.


text:
How is a stalling speed not affected by airspeed?
A question from the aerodynamics section of the FIA written:
Which action will result in a stall?
Incorrect answer: Flying at too low an airspeed
Correct answer: Exceeding the critical angle of attack
I could be flying in straight and level flight and if I slow down below stall speed the airplane will stall. So how can it be said that flying at too low an airspeed will not result in a stall?

related: http://aviation.stackexchange.com/q/6366/1467

If you reduce power, you have to pull back on the yoke to maintain altitude. As you slow, the angle of attack increases. It's the angle of attack that causes the stall, even though the reason for the angle being so high is lack of airspeed.

Thanks @Steve that is an easy-to-follow and succinct explanation. Those two qualities also make it rare for an answer on this site.

Yeah, now if only I was a pilot...

Flying at too low an airspeed will result in a stall, but what "too low" is entirely depends on the situation. For example, if you try a very steep climbing turn, you can stall at an airspeed where you would be fine in level flight.
The answer is an incorrect answer, not because it's factually wrong, but just because it's not general enough. Exceeding the critical angle of attack always causes a stall, and it is the cause of every stall, so it's the best answer to the question. The examiner wants you to understand that the angle of attack is the mechanism that causes stalling, and that it can happen at different speeds.
It's quite common for these (and other multiple choice) exams to have an "incorrect" answer that isn't completely wrong, but isn't as good as the correct answer because it only applies in some conditions or is less general in some other way. In this case, another "incorrect" answer would be "pulling the stick back too far". That's an action that might result in a stall, if you're already at a high angle of attack, but it's not really the cause and isn't always true.


text:
What is the difference between ADS-B UAT and ADS-B ES?
Why would an owner decide to put in an ADS-B UAT transceiver versus an ADS-B ES transceiver?  What are the advantages and disadvantages of both systems?

Do you mean 'transceiver'? Or perhaps transmitter?

Both could be accurate depending on the situation. Based on the OP's use of the term 'transceiver' in reference to both ADS-B systems, the assumption can be made that he would prefer those answering his question to provide a comparison of these two types of ADS-B systems where each are installed with the requisite components and properly configured with all functions of a combined ADS-B In/Out system available for use.

ADS-B UAT operates on 978 MHz and are available for aircraft that operate below 18,000 FT MSL.  UAT transceivers can display aviation weather products (FIS-B) as well as traffic from ground radio stations (TIS-B).  The airplane must be in a coverage area for TIS-B services.

TIS-B is intended to provide ADS-B equipped aircraft with a more complete traffic picture in situations where not all nearby aircraft are equipped with ADS-B Out. - AIM 4-5-8

Aviation weather products available over FIS-B through ADS-B UAT transceivers

ADS-B 1090ES operates on 1090 MHz and is required for aircraft operating above 18,000 FT MSL.  The information sent over a Mode S transponder is greater and is called an "extended squitter".  These aircraft will be able to display traffic directly from other ADS-B equipped aircraft and those with Mode S transponders without the need to rely on ground based systems.  FIS-B is not available to these transceivers.

The picture below illustrates where 1090ES and UAT are allowed / required for aircraft after the ADS-B mandate in 2020.

References:

Aeronautical Information Manual 4-5-7 thru 4-5-9
Advisory Circular 90-114A

Does they both same physical layer? I mean they (978UAT 1090ES) both have same modulation, like ppm (pulse pain shifting) for example?

@mohammadsdtmnd No clue.

I've seen some where that UAT uses continuos phase FSK while ES uses PPM, but I'm not sure!

If the aircraft will be flown above 18,000ft or in other countries with an ADS-B mandate, 1090ES is required.  This applies to pretty much all "large" aircraft; they have no choice.
However, the US has an enormous light GA fleet with very vocal owners, and the FAA itself was concerned about 1090MHz congestion, so they added UAT (on 978MHz) as an alternative.  It was assumed this would be simpler and less expensive than replacing every transponder in the entire GA fleet (which seems to be true), and they also added free weather (FIS-B) to encourage GA owners to choose UAT unless they actually need 1090ES, in case the price difference wasn't enough.
Note that both options offer TIS-B/ADS-R.


text:
How does turbojet thrust change with altitude?
I am studying the rate of climb of turbojets, I don't know how the thrust varies with altitude and in particular which are the differences between necessary thrust and effective thrust.
Hope being clear, I can't translate properly maybe so ask and edit !

The thrust variation with altitude would be highly engine specific, but the general trend is nicely depicted in the image below:

Read this for further details.

Thrust is produced by accelerating air. The exhaust air leaves the engine nozzle at a fairly fixed velocity, so the acceleration is mainly controlled by the difference between the exhaust and incoming airspeeds. The faster the aircraft speed, the less acceleration is being created on the intake air. Therefore thrust decreases with increasing airspeed.
In level flight, an aircraft could not be accelerated above the engine exhaust gas speeds because then the incoming air would be faster than the exhaust air - resulting in a decelaration.
Most jets are operated as max speeds much less than their exhaust gas velocity - about half of V-exhaust I think.
Since higher air-speeds are normally used at higher altitudes, thrust generally decreases with an aircraft's altitude. But increasing altitudes can erode thrust because the mass of air being consumed is also decreasing as the air gets increasingly more rarified.
To summarize: At a constant airspeed (ignoring airspeed measurement nuances) thrust decreases as air density decreases with increasing altitude; at a constant altitude, thrust decreases with increasing airspeed.
Note that max thrust is found when stationary, just before take-off, since that's when incoming air is at its slowest. Also, at max thrust, (when airspeed is zero) there is no work being done and hence the power produced is zero right before takeoff.

To make the answer more complete, you could add that flying faster increases compression in the intake and lets the whole engine operate at a higher pressure level, which offsets the decreasing speed difference effect especially at higher speeds and in low bypass ratio engines.

Also, at max thrust, (when airspeed is zero) there is no work being done and hence the power produced is zero right before takeoff. Are you using takeoff as the start of the takeoff roll? Otherwise there is some airspeed because the plane is moving.

I'm speaking in generalities; if a jet is stationary, the incoming air still has a non-zero velocity. And as Peter points out, there are compression factors, but I think the first-order effect is caused by airspeed.

As an aircraft increases in altitude, the density of air also decreases. Thrust available = Q(V1 - V2). The important part of this equation is Q. Q = (rho)AV which means that the smaller rho (density ratio) becomes, the less mass airflow there is which results in lesser thrust at high altitudes.

could be a possible duplicate of this question, http://aviation.stackexchange.com/q/16727/16812.


text:
Why do some Turbofan engines have multi-stage fans?
I was wondering why some turbofan engine have multi-stage fans.
e.g. F-119 engine:

F100

Saturn AL-31

All three engines you show are for fighter jets flying at supersonic speeds. These engines need a low bypass ratio: Why do military turbofan engines use a low bypass ratio?
To achieve more thrust at lower bypass ratios, the bypass air needs to be accelerated more, which requires more than one fan stage because axial compressors can only add a limited pressure difference per stage:

Low bypass engines often have a multistage fan which generates a low volume but relatively high speed air stream whereas high bypass engines usually have a single stage fan which generates a high volume but relatively low speed air stream.

(SKYbrary - Turbofan Engine)
In principle, high bypass ratio engines where more air is accelerated less are more efficient and therefore used on modern airliners, but they are not suited for supersonic flight.

Supersonic is not the driver, but the basic idea in the answer is correct. Fighter planes do not fly supersonic most of the times, and for supersonic speeds most planes require the afterburner (excluding supercruising fighters). Note that the engine in afterburner mode is basically an engine operating in max dry power with enabled afterburner and opened exhaust nozzle. Upstream of the afterburner, up to the front of the fan, the engine doesn't even know it's flying supersonic.


text:
What does "military band" mean in this chart?
I'm reading the manual of J79 engine, and I just saw this chart:

Does anybody know what "military band" means?

Possibly related

It's the part of the curve where the brass instruments play Sousa marches of course!

Military Power is the maximum rated thrust from the core engine, that is, without afterburner.  The Throttle Angle can vary within a few degrees when the engine is at Military Power due to tolerances in the fuel control system.
On a chart this variability (70-75 degrees) shows as a strip or band marking the minimum and maximum values of throttle angle.  Therefore, the "Military Band" represents the range of angles you can expect to see the throttle when the engine set to produce Military Power, or maximum non-afterburner thrust.

The F-4 Phantom throttles have an idle position and a military power position. This diagram shows nozzle position and diameter, as well as RPM at military power, otherwise known as the "military band". The reason the lines flatten out is that above that the throttles control the activation of the afterburners, but do not change the RPM of the engine itself or the nozzle configuration. There is a copy of the Phantom flight manual here that explains it in more detail.


text:
What is the difference between ICAO Annexes and Documents?
I have gone through many ICAO and aviation related sites, but I still can not understand the basic difference between annexes and docs.
What is it?

An Annex to the Chicago Convention on International Civil Aviation (which established the ICAO) is adopted by the ICAO Council according to particular voting procedures laid out in Article 90 of the Convention, and is binding upon the signatory States.
An ICAO document is just something the ICAO organization has chosen to publish in its Doc Series with an identifying number for ease of reference. Being an "ICAO Document" doesn't in itself say anything about who wrote it or its legal force. For example, Doc 7300 is the Chicago Convention itself, which has a lot of force (but not because it has a Doc number), whereas Doc 9562, Airport Economics Manual is more informative in nature and tries to be persuasive rather than authoritative.

Actually an Annex contains SARPs (Standards And Recommended Practices) that are to be adopted (Standards) or that member states should try to adopt (Recommended Practices). However documents contain further explanation and methods to implement SARPs.

An annex is quite literary what it says.  It is a document that is annexed (or "appended' or "attached") to the ICAO convention (where "convention" means a document). An "annex" states the Standards and Recommend practices (SARPs) expected from State signatories to the ICAO convention. The documents expand on the SARPS explaining and/or nominating how and what a State  has to do to achieve the SARPs. In essence in simplistic terms an annex is the chapter heading of a book and the documents are the content of the chapter.


text:
Does aircraft/ATC get any information directly from a weather balloon?
We have been launching radiosonde (a meteorological instrument containing GPS and other sensors to measure meteorological variables attached to a balloon filled with Hydrogen) to obtain meteorological data of the atmosphere.
This radiosonde transmits the information to a ground station we have in a frequency close to 400 MHz (well, this is what we uses, not sure about others). We frequently update the vertical height, direction, bearing position etc to our nearest ATC.
I was told that ATC and aircraft get information from the radiosonde directly (I am not sure what sort of information they get) other than what we give to the ATC.
So, I want to know whether ATC/aircraft get information directly from a radiosonde and if they do, what sort of information do they get?
I am not concerned about the radiosonde's meteorological data going into the weather models for forecast products that is available to aviation industry. What I need to know is if ATC/aircraft can know the position of the balloon/radiosonde directly by the information the radiosonde transmits?
In essence, can ATC/aircraft track a radiosonde without inputs from the agency that launches/manages the radiosonde?

Are you asking about tracking or location information for traffic separation, or metrological information?

@JWalters Specifically, tracking/location information, not meteorological info. In general, anything an ATC/aircraft gets from a radiosonde.

Ok, suggest you edit you question to make that clear since the current answers presume you are asking about weather information.

No, neither ATC nor aircraft receive any information directly from a radiosonde balloon. However, in many cases the meteorological agency operating the radiosondes does use them to produce forecasts that are made available to ATC and pilots.

Aircraft and ATC get "weather products" in various formats via communications systems that are standardized worldwide. National authorities build these products (or don't, in some areas) based on thousands of different data points collected over a large area. You can see examples of these products at the US National Weather Service's Aviation Weather Center; your country (you don't say where you are) hopefully has something equivalent, but possibly not freely available to the public or as comprehensive.
Your radiosonde is presumably providing additional data points into your national weather network, which will make their standard weather products in that area more up to date and more accurate. This benefits everyone that uses those weather products, including non-aviation users such as TV and radio stations, without having to know the vagaries of all the different systems (such as your radiosonde) collecting raw data behind the scenes.
ATC probably doesn't care what your balloon is doing per se, just that it's an airborne hazard they need to keep other aircraft away from, and that means they need to know where it is to do their job properly.

Does your radiosonde have an ADS-B transponder in it?  If so, then ATC and aircraft can get location and speed information from it without any other communication links.

Asking for clarification must be done in comments, not in answers. You should either ask for the ADS-B on the comments (so that the OP can precise its question) or put in your answer examples of balloon described in the question either fitted or not fitted with transponder. See the help center for more information.


text:
What's the point of writing on a cockpit window?
Why would a pilot write on the cockpit window like this?

From: source

I think this is used mostly by SAR or other military pilots when running grid work.

@RonBeyer: that makes sense, since the picture is about the search for the missing Malasia flight. So, normal commercial pilots would not do this?

poor man's HUD.

Welcome to Aviation.SE! Please post the source of your image.

@FreeMan: done, but it's in Greek.

@PierreB so what? You should always post it. ML knows how to (or at least give a (bad) approximation of a translation). Moreover, I am Greek, I can help if someone wants. iEfimerida stands for electronic newspaper.

"Squadron leader Brett McKenzie marks the name of another search aircraft on the windscreen of a Royal New Zealand Air Force P-3K2 Orion aircraft searching for missing Malaysian Airlines flight MH370 over the southern Indian Ocean. Photo: Reuters" (source)

@PierreB you may include in the question the context (I suppose it is explained in the paper linked, but extracting the relevant part could help understand the question's context)

If you use a grease pencil, it rubs off easily from glass
So you can save paper, not have to look down or hunt for the info, it won't fall on the floor, and if it's written in the center of the windshield, both pilots can see it

authors: Γιαννης Νικολοπουλος


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Which MEL categories would ground a commercial airliner if they're inoperative?
Which Minimum Equipment List (MEL) categories would require a commercial airliner to be grounded (Aircraft On Ground or AOG) if they're inoperative? For example, any category A MEL item is considered an AOG item; is it the same for category B items?

Any inoperative equipment not listed in the MEL caused the aircraft to not be airworthy... or AOG.

Not clear why the vores to close -- the question is quite straightforward, even if it reflects a misunderstanding of how the MEL works. It's a perfectly reasonable learning point for others with a similar question/misunderstanding.

The name is confusing. The MEL is a list of equipment that are allowed to be inoperative (and conditions and procedures concerning that). It is not the minimum list of essential equipment that must be operational.

FAR §91.213 requires that all equipment installed on the airplane is in an airworthy state.

(a) Except as provided in paragraph (d) of this section, no person may take off an aircraft with inoperative instruments or equipment installed unless the following conditions are met:

(1) An approved Minimum Equipment List exists for that aircraft.
(2) The aircraft has within it a letter of authorization, issued by the FAA Flight Standards district office having jurisdiction over the area in which the operator is located, authorizing operation of the aircraft under the Minimum Equipment List. The letter of authorization may be obtained by written request of the airworthiness certificate holder. The Minimum Equipment List and the letter of authorization constitute a supplemental type certificate for the aircraft.

Any inoperative equipment not listed on the MEL has to be operational before flight.
The purpose of the MEL is to provide relief from the requirement that everything works on the airplane to be able to legally takeoff.

Although all equipment listed on the MEL can be deferred there are specific conditions that have to be followed to ensure the flight can be operated safely.
For example take this excerpt from a commercial aircraft MMEL.

The crew can dispatch provided they mark the lavatory inoperative and lock the door.

The deferred equipment can't be deferred forever.  Notice the "C"; that is a repair category and it indicates how many days dispatch can occur without fixing the inoperative equipment.
Repair categories

A:  Time frame listed by the inoperative equipment
B: 3 calendars days before repair is required
C: 10 calendar days before repair is required
D: 120 calendar days before repair is required

If a piece of inop equipment CAN be deferred using the MEL, then the aircraft isn't grounded. When there is no MEL relief & the part has to be changed out is when you have an AOG scenario.


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Has any flight crew ever had to ask "Does anyone on board know how to fly a plane?"
Having just watched the classic movie 'Airplane' I'm curious if any crew of a large(ish) passenger aircraft has ever had to make a P.A. announcement asking for someone to fly a plane, and if so, what was the passengers reaction?
I'm not asking if any non-pilots have ever landed a plane, I'm curious about if there's ever been a situation where pilot and co-pilot have both been incapacitated and crew have had to identify an off-duty pilot amongst the passengers.

Related:https://aviation.stackexchange.com/q/11677/36893

This could happen if one of the pilots is incapacitated. While one pilot should always be able to safely fly and land an airliner, having a second person to handle the radio and other things can really help with workload, even if that second person is not qualified to fly that particular type of airplane.

Very related: Can a passenger realistically replace suddenly incapacitated pilots?

Of course you have the easy cases of crews with more than 2 pilots (very long flights have relief pilots), with a check captain or supervising check captain (see QF32), pilots deadheading or just flying gratuitously. Depending on the emergency, other qualified pilots on board may notice and offer their help spontaneously.

A PA announcement of that type would simply generate a panic reaction, which is exactly the opposite of what you want in that moment. Instead, in such cases (with both pilot and copilot incapitated) a crew member will normally take over. Off the top of my head I remember the Helios Airways Flight 522:
"At 11:05, two F-16 fighter aircraft from the Hellenic Air Force 111th Combat Wing were scrambled from Nea Anchialos Air Base to establish visual contact. They intercepted the passenger jet at 11:24, and observed that the first officer was slumped motionless at the controls, and the captain's seat was empty. They also reported that oxygen masks were dangling in the passenger cabin. At 11:49, flight attendant Andreas Prodromou entered the cockpit and sat down in the captain's seat, having remained conscious by using a portable oxygen supply. Prodromou held a UK Commercial Pilot Licence but was not qualified to fly the Boeing 737. Prodromou waved at the F-16s very briefly, but almost as soon as he entered the cockpit, the left engine flamed out due to fuel exhaustion and the plane left the holding pattern and started to descend. Crash investigators concluded that Prodromou's experience was insufficient for him to be able to gain control of the aircraft under the circumstances. However, Prodromou succeeded in banking the plane away from Athens and towards a rural area as the engines flamed out, with his actions meaning that there were no ground casualties." (emphasis mine).

I'm not aware of any examples, and it shouldn't really be needed.
The crew should be aware of any company pilots on board, even off duty and out of uniform, and those are the first choice since they'll already know the company procedures.  Example: Air Transat 186.
The second choice would be any other airline, charter or military pilots on board, whom the cabin crew could discreetly identify if they're in uniform.  Example: Southwest 6013.
I've discussed the scenario with some airline pilot friends, and their unanimous opinion is that it'd be safer to continue the flight alone than to invite an unknown pilot, who probably lacks professional CRM training and may be more of a hindrance than a help, into the flight deck.
If both pilots are incapacitated, it's probably a pressurization issue that would incapacitate all the passengers as well, so there is no help to be had anyway or anyone conscious to ask for it.  Example: Helios 522.


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Why are jet engine fan blades painted?
I came across this extremely interesting article with details on a 2018 United Airlines Boeing 777  engine failure, which is also related to the more widely-publicized failure a few days ago: http://aerossurance.com/safety-management/ndi-failures-b777-pw4077-fbo/ . Amongst others, it contains the following:

The records for the TAI [Thermal Acoustic Imaging] inspection in July 2015 as well as an earlier
TAI accomplished in March 2010 revealed a thermal indication in the
same location as where the LCF crack occurred. The records for the
fractured fan blade’s July 2015 TAI inspection was
annotated ‘paint’ that, according to the inspector, was consistent
with him accepting the indication because he thought it was an issue
with the paint.
Flaking paint was a regular issue, affecting perhaps 25% of blades,
requiring either a touch-up or a complete re-paint.

which begs the question: why are fan blades painted? The only reason I can think of is so that damage from small objects (or birds) sucked into the engine is easier to see? But, if I read the article correctly, the paint can also mask more serious issues - in this case, the inspector thought that the TAI indication was due to flaking paint, which is apparently common, when in fact it was a defect in the blade. So maybe it would make sense to leave the blades unpainted?

This seems like an attempt to devise a new inspection process. Given the amount of effort that is put into creating a highly polished surface on fan blades (at least by other engine manufacturers), painting them and not removing the paint seems crazy. There are reliable methods of detecting internal cracks that don't rely on inventing Rube Goldberg technology.

Note, fan blades are often painted with colored stripes when running a blade-off test, simply to make it easier for computer software to track the failure sequence from high speed video. But in that situation losing a few percent of aerodynamic efficiency is irrelevant.


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Are there full scale helicopters capable of sustaining inverted flight?
Background
I'm a model aviation enthusiast and really enjoy seeing how far people can push the limits of aerobatics. Model aircraft are able to push the limits well beyond full scale due to their small size. I understand that this is only possible because models are small* so they can withstand forces that would kill any occupants in a manned craft. Also, model aircraft are typically overpowered when compared to full scale aircraft
For example, I don't have a model plane that isn't capable of a 30 degree climb, and it's quite common for aerobatic model airplanes to have thrust to weight ratios exceeding 1.5:1 or even 2:1.
Question
When I first saw some of what model helicopters were capable of I was blown away. Having also seen some full scale helicopter aerobatics (Red Bull Demonstration) I've been wondering just what full scale helicopters are capable of.
In the Red Bull Demonstration it looked like the helicopter might use some negative (or at least neutral) collective while rolling, however it did not appear that it would have enough to sustain an inverted hover or inverted flight. This got me wondering: has there been a full scale helicopter capable of sustaining inverted flight or an inverted hover (which takes even more power)? If there hasn't, would it be possible to purpose build a helicopter that would be capable of doing so?
Note
I'm just wondering about technical possibility of creating a machine capable of this maneuver. For the scope of this question, please ignore the regulatory, legal, and economical obstacles that would need to be overcome to attempt such a feat (not to mention finding a pilot willing to try).
Also, I'm only asking about full scale inverted flight. I understand physics get in the way very fast if someone were to try to design a full scale helicopter capable of all the aerobatics models do.
*some model airplanes and helicopters can be greater than 35% full scale, but that is still small when compared to full scale aircraft.

I believe that the AH-64 can roll 360° with no forward airspeed, loosing altitude in the process. Either I've seen it or an illusion. I cannot find any evidence on Youtube, however. I would love for someone to either correct me or post video evidence, I've been wondering what I've seen for years.

Bear in mind that designs don't scale with the same power of their linear size. It's not just occupant killing that is the concern. Never mind acrobatic tricks, a faithfully scaled up model aircraft probably would be torn apart by the most boring of attempts to fly.

There is no real helicopter capable of sustained inverted flight and certainly not of hovering inverted.
The simple reason is that there is no operational need to do so.
Is it theoretically possible?  Yes, but you have a lot of engineering problems to overcome.  In addition to inverted fuel and oil systems, the rotor head will be much more complex than it needs to be.
When flying normally, the fuselage effectively hangs like a pendulum underneath the rotor with a large nut (OK, it's more complex than that) holding the rotor onto the rotor shaft hub.  The rotor pulls the fuselage up.
To fly inverted, the rotor must be able to push the fuselage up so the hub would have to be engineered to apply and withstand the appropriate forces in both directions.
Then, the rotating blade cuffs, and associated hinges, must be designed so that the blades can have negative pitch (as seen from the normal attitude) across the full range.  In turn, this means that the collective mechanism must be designed so that it is possible to set positive and negative pitch as required.
The blades would also need to be more rigid, so that they do not cone excessively and contact the fuselage.  This would add more forces to the head joints which would need to be strengthened.
Finally, you must "reverse" the cyclic controls so that the expected collective pressure, cyclic back; nose up; reduce speed; works in the opposite sense.
I've probably not considered other factors.
All of this would add significant complexity and weight which, given that there is no requirement for inverted flight, will not happen.
By the way, the power required to fly or hover inverted would be the same as for normal flight.  There is no intrinsic reason why inverted flight would need more power - blades rotating at a certain speed, with a particular angle of attack will generate the same amount of lift no matter which way they are pointing.
The reason model helicopters can do this is that the ratio of the weight of the rotor to the weight of the fuselage is much lower than for a real helicopter, and therefore requires a lot less power to produce the required lift, and models have a much higher power to weight ratio.
The reason real helicopters can go inverted for a short time in aerobatic manoeuvres is because the momentum on the fuselage is pulling away from the rotor so that the lift vector pointing up, toward the fuselage, is countered. Consider an outside loop, which would be impossible, since the momentum of the fuselage and the lift vector would combine.
You should be able to demonstrate this in a model with a normally designed rigid head.  Add some (secure) ballast in the fuselage until you must accelerate in ground effect to gain translational lift in order to be able to climb out.  Now try to fly inverted!

That's pretty much what I figured. The lack of need combined with extra engineering required mean it just won't happen.

With regard to your point on power, would I be correct in assuming the blades have a symmetrical airfoil if they produce the same amount of positive or negative 'lift' for a given angle of attack?

@jdkorv11 Not necessarily, although many blades are symmetrical.  If not, all you need to do is increase the angle of attack.  Remember that power is needed to overcome drag which is a function of how much lift is produced.  To produce the same amount of lift from any given aerofoil needs the same amount of power, regardless of where the lift vector is pointing. Imagine a stationary blade.  You can make the angle of attack (pitch) whatever you like with no power since no speed = no lift = no drag.

Thanks for clarifying. I was operating on the assumption that the rotor was at speed and being supplied with the power necessary to sustain flight. I should have stated that. Also, are helicopters typically capable of providing some negative collective? I'd guess so but that's just a guess.

@jdkorv11 Not as far as I know.  All the types I've flown have slight positive pitch when the collective is fully down to control the rotor RPM in autorotation.  The ability to generate negative pitch without the stronger head and stiffer blades I mention in the answer would make the chances of a rotor strike higher.

Thanks for the insight. While I'm pretty comfortable with the models, I don't know about the challenges faced by full scale helis. For example, models typically don't need to worry about rotor strikes  so I've never really thought about those as problems before. Also aerobatic models will often allow as much negative collective as positive so that's why I was wondering about full scale.

@jdkorv11 You're welcome.

The pendulum argument does not work. In a pendulum the force at the hinge must be always upwards, but the lift tilts with the rotor and that tilts with the fuselage. Rotorcraft are not stable and higher or lower CoG does not change it (as it does not change roll stability in fixed wing aircraft).

I feel like the part about "Gravity will try to pull the fuselage down and sideways, the pendulum will try to swing down" might be suffering from something akin to the pendulum rocket fallacy.

So how did they shoot the scene near the end of Escape to Witch Mountain? This was in 1975, long before CGI effects.

@JDługosz I have no idea, you'd be better asking the film makers.  Unless you are suggesting that because CGI didn't exist, the helicopter must have flown upside down in which case I'd ask you how they got the van to fly?  Or any other number of special effects used.   BTW, the first use of CGI in films was in 1973 and there were many techniques used before that such as matting and compositing.

@JanHudec Actually, it does. I started to write some fairly long comments explaining why but it just kind of clogged things up so I'll simply delete the paragraph. I think I've done a reasonably good job of explaining why helicopters don't fly inverted. Vertical CofG certainly does affect roll stability, it simply isn't relevant in a fixed wing since it is not possible to move the CofG significantly, vertically, away from the aerodynamic centre. which is roughly where the wings are. In a helicopter, the CofG is significantly below the aerodynamic centre, at least when not inverted :)

@Simon, yes, it's complicated (for fixed wing aircraft it actually does matter for slip too; it does not matter for roll alone). Anyway, all the other reasons you gave are good ones.

@user2357112, yes, it does. But then it gets more complicated, because as the aircraft actually starts flying sideways, another aerodynamic force will appear and that will now be able to create torque. So there actually is some effect, but it can't be explained with pendulums.

I don't think the cyclic needs modification: a pilot could be trained to handle inverted flight. And I guess we'll have to convince some daredevil to actually modify a helicopter - as I see it, there's only practical considerations, no fundamental limitations.

@user2357112: The pendulum fallacy is the idea that mounting the lift generator above the CoG will lead to passive attitude stability. This is false; no matter whether the rotor is above or below the craft, you need some active control of the direction of lift to achieve stable flight. In a helicopter that's what the cyclic provides. Still, however, you need quicker and/or more precise control in order to be able to balance your craft atop the lift point than you need for dangling from it. That's easier to provide for rockets because they're not expected to hover anyway.

"Consider an outside loop, which would be impossible, since the momentum of the fuselage and the lift vector would combine."  By that logic, outside loops should also be impossible for fixed-wing aircraft.

Most full size helicopters can apply some negative collective; this is required to be able to autorotate on power failure (which all helicopters are expected to be able to do in order to be flight certified).  Having enough negative collective (along with all the other factors) to fly or hover inverted is another thing entirely.

Back then I wrote that there is no pendulum, but that's not entirely true. The fuselage is hanging below the rotor pretty much exactly like a pendulum. The rotor is tilted by adjusting the aerodynamic forces it creates, but the fuselage can swing around below it. This still does not contribute stability for the abovementioned reasons, but it would create additional instability in inverted flight. In fact some designs have a problem, called mast bumping already at low positive loading.


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To what requirements / assumptions RSAs & EMAS designed?
I've several questions regarding runway safety area (RSA) and engineered materials arresting system (EMAS):

Is RSA/EMAS designed for impact loading at the point of touchdown and to absorb energy of the moving aircraft for deceleration?
Assuming that the landing gear is down: Can the load be assumed to be distributed equally through all wheels?
Are the brakes applied to assist deceleration over the EMAS?
What happens in case of belly landing and when there are burst tyres?
Is there any other loading case?
What are the max design loads and velocities?

EMAS is designed only for runway overrun, not as an impact surface. Since only a small area of the runway has this material it would not help much in the event of a crash like that.

I will try to answer as much of this as I can, but there may be some holes to fill:

Is RSA/EMAS designed for impact loading at the point of touchdown and to absorb energy of the moving aircraft for deceleration?

No, the arresting materials is only there for a runway overrun condition. It is not designed to absorb an aircraft impact, nor is it designed for an aircraft to touchdown on it, crash or normal landing. EMAS area's are very specifically marked:

(Source: BusinessInsider.com)
As you can see, the area is actually very small, and the markings denote that this is not a touchdown zone. It would be very hard for a commercial airliner to hit that small patch on touchdown, purposely or not.

Assuming that the landing gear is down: Can the load be assumed to be distributed equally through all wheels?

Not always, because this depends on the loading of each aircraft gear. The load can be thought of as being distributed evenly probably at the same ratio as the weight on each gear, but only when all the gear are inside the EMAS area. This wouldn't be true, for example, as the aircraft enters the EMAS and the EMAS is acting only on the nose gear until the main gear exit the runway and into the EMAS.

Are the brakes applied to assist deceleration over the EMAS?

YES! The pilot is going to be braking hard to try to stay out of the EMAS, it is unlikely that they would release the brakes as they enter the EMAS. Remember that EMAS is basically very weak concrete. The pilot is probably going to be using all available braking (wheels, aerodynamic, thrust reversers, etc) to get the aircraft to stop. Once they enter the EMAS they've destroyed the EMAS pad, and have done significant damage to the aircraft.

What happens in case of belly landing and when there are burst tyres?

If a belly landing made it all the way to the EMAS, which is unlikely, I doubt that the EMAS will have a significant effect on stopping the aircraft. The EMAS is specifically designed to grab onto things that dig into it. By the time the aircraft slides onto the EMAS, whatever is "hanging" under the aircraft has probably been broken away. I don't know of any case where EMAS encountered a belly landing aircraft (other than one's who have broke off the nose gear), but I'm guessing that the aircraft would slide right over it.
Burst tires on the other hand would probably not reduce the effectiveness of the EMAS since it will still be dragging through the gear.

Is there any other loading case?

Not that I know of. The system is designed to arrest aircraft overruns. Specifically this means a normal landing but unable to stop on the runway such as in ice, snow, or water. It is not designed as a crash safety system.

What are the max design loads and velocities?

If you want to read the FAA's Advisory Circular on Engineered Materials for Arresting Systems (EMAS) for Aircraft Overruns it has quite a bit of information:

The design
  (or critical) aircraft is defined as that aircraft using
  the associated runway that imposes the greatest
  demand upon the EMAS. This is usually, but not
  always, the heaviest/largest aircraft that regularly
  uses the runway. EMAS performance is dependent
  not only on aircraft weight, but landing gear
  configuration and tire pressure. In general, use the
  maximum take-off weight (MTOW) for the design
  aircraft. However, there may be instances where less
  than the MTOW will require a longer EMAS. All
  configurations should be considered in optimizing the
  EMAS design. To the extent practicable, however,
  the EMAS design should consider both the aircraft
  that imposes the greatest demand upon the EMAS
  and the range of aircraft expected to operate on the
  runway.

FAA Advisory Circular AC150-5220 22b, 9.c
And this one:

Entrance Speed. To the maximum extent
  possible, the EMAS must be designed to decelerate
  the design aircraft expected to use the runway at exit
  speeds of 70 knots without imposing loads that
  exceed the aircraft’s design limits, causing major
  structural damage to the aircraft or imposing
  excessive forces on its occupants.

9.g from the FAA Circular, emphasis mine.